1. Field of the Invention
This invention relates to digital communication systems and, more particularly, to a coder arrangement which may be employed in such systems for coding orthogonally transformed image signals.
2. Description of the Prior Art
Various digital data systems are known for communicating image signals such as are associated with a television picture, some of which are concerned with reducing the signal transmission bit rate. Exemplary of such bit rate reduction systems are those of the type known as differential pulse code modulation (DPCM) systems. Typically, a DPCM system employs a predictive coder using a feedback circuit to provide a comparison signal so that only the difference between a previously coded image signal and an upcoming image signal need be currently coded and transmitted.
Efficient picture source coding can also be accomplished by first determining the structure of the pictorial data and then developing a coder that is efficient for data having that particular structure. Since pictorial data are not usually homogeneous, different regions of a picture often contain different structures. Accordingly, it is common to partition a picture image into subpictures, each having a plurality of picture elements, also called pels in the art, and to design a coder for the data structure of a subpicture.
In addition, it is known that the pel signals may be transformed by orthogonal transform logic to provide a plurality of transform coefficient signals. Related thereto is a Hadamard orthogonal transformation, which is known to offer still another means for reducing the bit rate of an image signal. Our invention broadly relates to the aforementioned prior art bit rate reduction arrangements, but more specifically improves upon orthogonal transform arrangements.
A typical Hadamard orthogonal transformation can be illustrated using a four-by-four Hadamard matrix. Using common matrix notation, the illustrative Hadamard matrix, arranged in order of increasing sequency, is: ##EQU1## Sequency is a common term of art relating to the number of algebraic sign changes in a row of the Hadamard matrix. Premultiplying the Hadamard matrix by a row matrix of, for example, pel signals A, B, C and D results in a row matrix of Hadamard transform coefficient signals H.sub.1, H.sub.2, H.sub.3 and H.sub.4 :
h.sub.1 = a+b+c+d PA1 h.sub.2 = a+b-c-d PA1 h.sub.3 = a-b-c+d PA1 h.sub.4 = a-b+c-d
that being the case, the first Hadamard coefficient signal of a subpicture image signal, H.sub.1, is seen to be equal to the sum of the four pel signals. Consequently, H.sub.1 proportionally contributes the largest fraction of bits necessary to define the subpicture. Thus a significant reduction in the transmission bit rate could be achieved by efficiently coding H.sub.1 as, for example, by using fewer bits. However, known coding arrangements typically code all coefficient signals including H.sub.1 using standard pulse code modulation (PCM) or DPCM techniques.
Accordingly, it is a broad object of our invention to provide an arrangement for efficiently coding an orthogonally transformed image signal.